This invention relates to a charged particle beam apparatus, or in particular, a scanning electron microscope for observing or measuring the shape, material, etc. of a sample utilizing a physical phenomenon such as the generation of secondary electrons obtained from a sample to which the charged particle beam is applied.
Secondary electrons can be generated by applying the electron beam to a sample in vacuum. The scanning electron microscope is used to observe the shape of a sample taking advantage of the phenomenon in which the secondary electrons thus generated undergo a change depending on the shape of the sample. Normally, the electron beam radiated is reduced by an electron lens having a focal point on the surface of the sample. By doing so, the roughness of the sample can be grasped as a clear image. The clarity of the image is determined by the degree to which the electron beam is reduced. Specifically, the image resolution can be improved and the image clarity increased by reducing the electron beam strongly. An excessive reduction, however, would exceed the performance of the electron lens and the resulting increase in the aberration such as the diffraction aberration would deteriorate the image clarity. In designing an electron microscope of a high resolution, therefore, the design of an optical system as a whole including a plurality of lenses is finally determined while reducing the electron beam by the electron lens called an objective lens placed in opposed relation to the sample to the degree just before the diffraction aberration begins to exceed the image resolution of the objective lens.
The reduction in the electron beam by the objective lens, though capable of improving the image resolution, shortens the depth of the clearly visible surface roughness, called the focal depth, of the sample in inverse proportion.
The semiconductor devices recently fabricated in the semiconductor fabrication process are inspected using this electron microscope, and with the ever increasing micronization of the fabrication process, an electron microscope of a higher resolution is in demand. As described above, however, an improved image resolution shortens the focal depth, and only those samples having a shallow roughness can be observed.
In the semiconductor fabrication process, on the other hand, the increased micronization is accompanied by the tendency to increase the depth of the holes often called the contact holes formed in the sample surface. In order to meet these contradicting requirements, U.S. Pat. No. 6,538,249B1 (Patent Document 1), for example, discloses a technique in which a plurality of SEM (scanning electron microscope) images are retrieved while changing only the focal point at the same position on a sample, and only the images in focus obtained by the subsequent image processing are combined, thereby finally forming the whole SEM image in focus regardless of the depth of the sample roughness.
JP-A-5-3013 (Patent Document 2), on the other hand, describes a process of inspection by focusing the image of a rough sample in accordance with the roughness under the scanning electron microscope.